In the spirit of the JPG lawnmower, and inspired by the blinkenlights people - make rooftiles with a hinge and something that can flip the tile up or flat down again. Under each tile, paint a white patch, or better still use a backlight. The root of each tile (that stays fixed while the tile moves) connects
to its neighbour and copies its on/off state every frame. Connect a computer to one end of it and scroll messages to entertain the passengers of passing aircraft. If everyone on your street / in your neighbour hood buys one and networks together (insert protocol design here), you can even display rude pictures.